Boiling apparatus for continuous crystallization and method of operating said apparatus

ABSTRACT

AN APPARATUS FOR CONTINUOUS CRYSTALLIZATION COMPRISING AN ELONGATED, HORIZONTAL EVAPORATING CHAMBER HAVING AN INCREASING TRANSVERSE SECTION AND AN INCLINED BOTTOM FROM ONE END TO THE OTHER. A CONSTANT LEVEL BATH OF MOTHER SOLUTION CONTAINING THE GROWING CRYSTALS IS HEATED BY A LONGITUDINAL SERIES OF HEATING ELEMENTS INCLUDING HEATING SURFACES WHICH INCREASE LONGITUDINALLY AND FORM, ON ONE SIDE OF SAID CHAMBER A SERIES OF VERTICAL AND TRANSVERSE PASSAGES. THE CRYSTALS IN SUSPENSION IN THE BATH ADVANCE LONGITUDINALLY BY GRAVITY DURING THEIR GROWTH, WHILE FOLLOWING A HELICOIDAL PATH WITH CONSTANT PITCH.

Jan. 12, 1971 A. JAVET ETAL 3,554,800 BOILING APPARATUS FOR CONTINUOUSCRYSTALLIZATION AND METHOD OF OPERATING SAID APPARATUS Filed April 30,1968 2 Sheets-Sheet 1 Jan.-12,1'9 71 MVQ ETAL "3,554,800

BOILING APPARATUS FOR CONTINUOUS CRYSTALLIZATION AND METHOD OF OPERATINGSAID APPARATUS Filed April 30,1968 2 SheetsS h'eet 2 Vo le/ 20 2/ 20 2/20 U 2 0 2 50 52 26 It A w 1 r .JJS

, -L L L A; -4 #4 J4- United States Patent 3,554,800 BOILING APPARATUSFOR CONTINUOUS CRYS- TALLIZATION AND METHOD OF OPERATING SAID APPARATUSAlain Javet, Grange-Canal, Geneva, and Aldo Torelh, Grand-Laney, Geneva,Switzerland, assignors to Ateliers Belges Reunis S.A., Petit-Enghien,Belgium Filed Apr. 30, 1968, Ser. No. 725,322 Claims priority,application Switzerland, May 2, 1967, 6,332/ 67 Int. Cl. C13f 1/02, 1/00US. Cl. 127-16 8 Claims ABSTRACT OF THE DISCLOSURE DISCLOSURE Theprocess of crystallization by growth of seed crystals in a bath ofsolution has been the subject of numerous studies which havedemonstrated that crystal growth is influenced by a great number ofparameters which are difiicult to control on an industrial scale so asto obtain a uniform rate of crystal growth for all crystals.

In the sugar industry, crystallization is generally carried out byboiling mother solution containing seed crystals in suspension, whilecontinuously supplying feed solution so as to maintain a degree ofsupersaturation allowing progressive crystal growth. It is known that inorder to obtain uniform crystals, the degree of super saturation of themother solution must be maintained while boiling, in the zone ofsupersaturation known as the metastable zone and in which existingcrystals may grow rapidly but in which no new crystals can form. It isfurther not only necessary that crystal growth be effected with seedcrystals having the same size but also that the duration and conditionsof growth be as far as possible the same for all the crystals in orderto obtain a uniform product. These conditions are however difficult tofulfill as the mother solution containing the crystals is generallyagitated to a great extent while boiling thereby mixing crystals atdifferent stages of growth with one another.

The boilers generally employed for crystallizing sugar generally operatediscontinuously. As the syrup is supplied continuously in thesugar-refinery, discontinuous boiling requires the use of buffer tanksand waiting mixers before the separators. A further disadvantage ofthese discontinuous boilers is their relatively great bulk with respectto capacity. In addition, discontinuous operation requires regulatingthe different parameters which determine crystal growth while boiling.Thus for example, the heat supplied should be adapted at every instantto the increasing amount of mixture of crystals and mother solutionpresent in the boiling apparatus. As the heating surface is constant,this regulation poses considerable problems.

Certain known boilers for crystallizing sugar comprise mechanical meansfor effecting forced circulation in the Patented Jan. 12, 1971 bath ofmother solution to maintain uniform conditions therein as far aspossible. It is well known that mechanical circulating devices poseconsiderable practical problems when used in a boiling apparatus,especially in a sugarrefinery boiler. It may be mentioned that, in thiscase, a motor with a speed-reducing gear, must be provided outside theboiler for driving a screw or propeller arranged in the bath of mothersolution. Special packings are thus required for the driving shaft andthese must be capable of functioning at the boiling temperature and ofpreventing the infiltration of any mother solution and hence theformation of crystals therein, in particular when cool, e.g. whenboiling stops. In addition, especially when using a screw of greatlength, it is necessary to mount bearings within the boiler itself andthis likewise creates problems. Besides the difficulties which arebrought about by using packings and bearings operating at hightemperature in the presence of viscous liquids, these mechanical meanscontinuously conduct heat out of the boiler and considerable insulationproblems must be solved in order to prevent formation of crystallinedeposits on the agitating surfaces. Experience has shown in fact thatmechanical circulating means are hardly suitable for use in boilers andin boilers used in sugarrefineries in particular.

A known apparatus for continuously crystallizing sugar comprises anelongated, substantially horizontal evaporating chamber subdivided intoa longitudinal series of communicating compartments equipped withheating elements. This apparatus further comprises feed means anddischarge means for forming a bath of mother solution supplied with feedsolution along its entire length, containing growing crystals insuspension and advancing horizontally from one end of this chamber tothe other by gravity, while circulating transversely by convection.These successive evaporating compartments, each supplied with feedsolution, thus correspond to several evaporators operating in series andserving, in particular, to reduce mixture of the massecuite or mass ofsyrup and crystals at different stages of growth of the crystals. Thevolumetric flow rate of the massecuite increases, however, along theapparatus and, the cross-section of the latter being constant, the speedof advance of the bath increases likewise. In addition, the viscosity ofthis mass also increases considerably during growth of the crystals andthus affects heat exchange as well as the transverse circulation bynatural convection. Circulation is eliected, in this known apparatus, inseveral inner ascending branches connected to two lateral descendingbranches. This results in variations in the transverse circulating speedand the formation of undesirable stagnant zones, particularly where themass coming from the outer descending branches passes to the innerascending branches so that the crystals may decant prematurely or bemixed with crystals at other stages of growth in these zones. Such acirculation also produces considerable pressure loss.

An object of the present invention is to overcome the above mentioneddisadvantages by providing a boiling apparatus for continuouscrystallization wherein the circulation of the mother solutioncontaining the seed crystals takes place in a uniform manner andaccording to a well determined path.

The invention relates to an improvement of a boiling apparatus forcontinuously crystallizing a substance, in particular sugar, whichcomprises an elongated, generally horizontal evaporating chamberincluding heating elements and feed and discharge means for forming abath of mother solution supplied with feed solution along its entirelength, containing growing crystals in suspension and advancinghorizontally by gravity from one end of said chamber to the other whilecirculating transversely by convection. The improvement whichconstitutes the present invention lies in that the apparatus furthercomprises means for feeding seed crystals, arranged at the up-stream endof said chamber, said heating elements being arranged so as to form, onone side of said bath, at least one row of vertical passages whichextend transversely with respect to said advancing movement and are openat both ends for producing, by natural convection in said bath, anascending movement in said passages on one side of said chamber and adescending movement on the other, the heating surface of said elementsper unit length of said chamber .increasing along the latter, saiddischarge means being arranged at the down-stream end of said chamber soas to form an overflow for maintaining the level of said bath above saidelements, the bottom of said chamber being inclined and the widththereof increasing, in the direction of the advance of said bath, insuch manner that the cross-section of the latter varies proportionallywith the volumetric flow-rate passing through each cross-section so asto advance said bath at substantially constant speed whereby said bath,together with the growing crystals contained therein, is made to movealong a helicoidal path with substantially constant pitch providinguniform growth of the crystals up to the desired size.

The variation of the cross-section of the apparatus according to theinvention thus provides substantially uniform helicoidal movementthroughout the apparatus, with minimum pressure loss. It thus becomespossible to select a relatively small cross-section at the entrance ofthe apparatus so as to obtain a sufliciently high speed of advance toprevent mixing of crystals at different stages of growth. At the sametime it also becomes possible to adapt the heating surface to thevariable characteristics of the bath and, in particular, to vary theheight of these elements which depends on the increasing viscosity ofthe bath so as to obtain satisfactory transverse circulation up to theexit end of the apparatus. It is thereby possible to ensure effectiveheat and mass transfer by uniform rapid circulation of the bath whileavoiding the formation of undesirable zones in which either the degreeof supersaturation is too high thus favoring the formation of falsegrains or the concentration is too low thus leading to diminishment ofthe crystals. Experience has shown that movement of the bath along ahelicoidal path having substantially constant pitch is obtained even ifthe flow rate of the bath is varied substantially. Thus, owing to itsparticular construction, the apparatus according to the invention allowsseveral parameters influencing the process of crystallization to betaken into account and even eliminates the need for controlling them. Itis, in fact, sufiicient to maintain the bath at a constant degree ofsupersaturation by adjusting the quantity of heat delivered whichdepends on the seed crystal feed rate on one hand and on the feedsolution supplied on the other in order to obtain uniform crystal growthup to the desired size.

The invention also relates to a method of operating the apparatusaccording to the invention, said method comprising the steps ofintroducing into said evaporating chamber an amount of feed solution toform said constant level bath, heating this bath by means of saidheating elements to cause boiling so that the concentration thereofattains a value corresponding to a predetermined degree ofsupersaturation allowing said crystal growth, while maintaining in saidchamber, above said bath, a constant predetermined pressure, introducingcontinuously along said bath a predetermined amount of feed solution,introducing continuously into said bath a predetermined number ofcrystals, in suspension in a liquid, and adjusting the amount of heatdelivered by said heating elements which depends on the feed solutionflow-rate, whereby the solution forming said bath is kept boiling so asto maintain it at the desired concentration While this bath advanceslongitudinally at a constant predetermined speed.

The annexed drawing shows schematically an embodiment of the apparatusaccording to the invention as well as a variant thereof which isintended in particular for crystallizing sugar.

FIG. 1 is a longitudinal section of this embodiment.

FIG. 2 is a cross-section along line IIII of FIG. 1.

FIG. 3 is a horizontal section along line III-III of FIG. 1.

FIG. 4 is a longitudinal section of a variant of the apparatus accordingto FIG. 1.

FIG. 5 is a cross-section along line V-V of FIG. 4.

FIG. 6 is a cross-section along line VI-VI of FIG. 4.

As may be seen from FIG. 1, the apparatus comprises an elongatedevaporating chamber 1 arranged horizontally. This chamber comprises abottom 2 which is inclined downwards from one end (AA), towards theother end (BB which will be respectively called the entrance and theexit of the apparatus below. A plurality of inclined inlet tubes, in thepresent case six tubes 3 to 3 which are connected to a supply source P8of solution for continuously introducing into said chamber 1 anadjustable quantity of feed solution, are fixed to the inclined bottom 2in a longitudinal row. These inlet tubes 3 to 3 are provided with valves4 to 4 for respectively regulating the feed flow rates Q to Q so as tomaintain the bath of mother solution during crystallization.

Introduction of seed crystals from a supply source SC is effected in aliquid, in an adjustable amount by means of an inlet tube 5 providedwith a valve 6 and arranged at the entrance AA of the chamber 1. Theseed crystals must obviously be of substantially equal size in order toobtain uniform crystallization in the apparatus.

A discharge conduit comprising a horizontal branch 7, a vertical branch8 and a horizontal branch 9 is arranged at the exit BB for evacuatingthe crystals having the desired size in suspension in solution, eitherto an apparatus, not shown, for subsequent treatment, or to a reservoir.A transverse partition wall 10 is arranged a short distance before theexit BB and extends vertically in the chamber 1 from a point situatedabove the level of the horizontal branch 7 down to a point situated at adistance above the bottom 2 so as to form an ascending passage 11communications at its lower end with the bath of mother solution and atits upper end with the discharge conduit 7, 8, 9. This discharge conduitthus forms an overflow allowing said bath of solution to be kept at aconstant level (shown by dashed lines in FIGS. 1 and 2) on one hand andto cause the bath to advance by gravity towards the exit BB on the otherhand. The mixture of solution and crystals having the desired size whichis evacuated at the end BB is continuously replaced by supplying freshseed crystals at the entrance AA and mother solution along the bottom 2of the chamber.

As shown in FIGS. 1 to 3, the cross-section of the chamber 1, and henceof said bath, increases progressively between the entrance AA and theexit BB. This increase is due on one hand to the slope of the bottom 2and on the other to the increase in the width of the evaporating chamber1 and in such that the ratio between each cross-section and thevolumetric flow rate passing horizontally through this section bemaintained constant whereby the bath advances longitudinally at constantspeed towards the exit BB.

A heat exchanger 12 supplied with steam from a source V extendslongitudinally from the entrance AA up to the partition wall 10 situatedbefore the exit BB, this exchanger being immersed in the bath of mothersolution. As shown in FIGS. 1 to 3, the exchanger 12 comprises a steamchamber 13 supplied with steam by means of an inlet tube 14 connected tothe source V and equipped with a regulating valve 15, this steam chamberbeing arranged longitudinally in the evaporating chamber 1, in thevicinity of the side wall 16 thereof. The heating surfaces of theexchanger 12 consist essentially of a series of vertical transversewalls 17 connected in pairs by vertical, longitudinal walls of slightwidth,

thereby forming a longitudinal series of transverse passages 18 eachcommunicating at both ends with the bath of solution.

The heat exchanger 12 serves to maintain the mother solution boilingwhereby vapor bubbles of solvent are formed in the vicinity of theheating walls 17 forming the passages 18. These bubbles rise therebyproducing an ascending movement of the solution in these passages 18 andare released at the surface of the bath. The bubbles are then evacuatedthrough a discharge orifice 19 which, in the present case, is connectedto atmosphere. As may be seen from FIGS. 1 to 3, the heating surface ofthe heat exchanger 12 per unit volume of the chamber 1 increasesprogressively along the latter, so as to accommodate the increase involume of the bath towards the exit BB as well as the decrease in theheat-transfer coefiicient, due to the increase in viscosity of themixture of solution and crystals towards the exit, so that the bathascends at a constant speed along the entire apparatus.

The heating passages 18 thus serve to produce a transverse circulationconstituted substantially by said ascending movement on one side of thechamber 1 and said descending movement on the other side, thiscirculation being represented by arrows in FIG. 2.

A suitable adjustment of the feed flow rates Q to Q and of the flow rateQ of the steam delivered to the exchanger 12 enables the bath to advanceat constant horizontal speed towards the exit BB, as well as saidtransverse circulation of this bath by natural convection, whereby thebath and the seed crystals contained therein move at constant speedalong a helicoidal path with a well determined substantially constantpitch. The stable flow thus obtained allows mixing of crystals ofdilferent ages to be largely prevented. Consequently, seed crystalshaving the same initial size all follow the same path and hence aremaintained for the same period in the bath so that they grow uniformlyand will. attain substantially the same size at the exit BB of theapparatus. It is, however, obvious that in spite of these substantiallyuniform flow conditions, certain seed crystals, and especially thelargest ones, may grow more rapidly than the others. Any crystals whichattain prematurely a size and thus a mass which is too great to allowthem to be carried upwards by the mother solutions will decant onto theinclined bottom 2 and will advance by gravity along the latter towardsthe exit BB. Thus the uniform circulation on one hand and the relativelyrapid evacuation of the large crystals along the inclined bottom 2 onthe other, provide crystals of substatnially uniform size at the exit ofthe apparatus.

It should be noted that, in this embodiment, a suitable adjustment ofthe heat supplied, i.e. of the steam rate Q on one hand and of the fieedflow rates Q, to Q on the other allows a constant degree ofsupersaturation to be maintained in the bath of mother solution. To thiseffect, the supply of feed solution must obviously compensate the effectof evaporation as well as crystallization so as to maintain a constantdegree of supersaturation of the mother solution throughout the bath.

The modified boiling apparatus shown in FIGS. 4 to 6, comprises anevaporation chamber similar to that described above and shown in FIGS. 1to 3. In addition to the heat exchanger 12, it comprises a plurality oftransverse partition walls 20 for subdividing the bath of solution intoa longitudinal series of compartments 21. In the present embodimentshown, six partitions walls 20 and seven compartments 21 are provided.The successive compartments 21 communicate with each other by means ofopenings 22 arranged in the partition walls 21, just above the bottom 2.

Thus the mother solution containing the growing crystals follows thesame helicoidal path with constant pitch as described above whilepassing from one compartment 21 to the next through the opening 22 nearthe bottom 2. It is thus obvious that the partition walls serve solelyto render movement of the solution forming the bath more uniform forpreventing any possible backward movement and consequently mixing ofcrystals of different ages.

As further shown in FIG. 4, the vapor discharge orifice 19 is connectedto a discharge conduit 23 provided with a valve 24 and leading to acondenser 25. The latter is connected in known manner to a vacuum pump26 and supplied with water by means of a conduit 27 provided with avalve 28, whereby a desired sub-atmospheric pressure may be maintainedin the evaporating chamber 1. Thus, in this modified embodiment, thepressure prevailing above the surface of the bath, in the evaporatingchamber 1 may be controlled, so that the boiling temperature of the bathmay be adjusted to a desired value substantially lower than the boilingpoint at atmospheric pressure.

As shown in FIG. 6, the heat exchanger 12 of the modified boilingapparatus comprises a vapor chamber 13 divided by a partition wall 29into two sections 13a and 13b. The latter are respectively fed Withsteam from source V by means of lines 14a, 1417 provided with regulatingvalves 15a, 15b. This division into two sections allows the amount ofsteam delivered to these two sections to be varied individually so as totake into account the need for evaporating a relatively large amountofwater in the last section 1315 with respect to the first section 13a.

As may further be seen in FIGS. 4 to 6, a heating jacket 30 surroundsthe inclined bottom 2 of the evaporating chamber 1 as well as the sidewall 16 in the vicinity of which the exchanger 12 is arranged. Thejacket 20' is supplied with a suitable heating medium such as, forexample, the condensed steam leaving the exchanger 12 and allows thetransverse circulation to be improved and the formation of crystallinedeposits on the bottom 2 to be prevented.

It may be mentioned that reducing the boiling temperature providescertain advantages with respect to boiling at atmospheric pressure.Thus, for example, such temperature reduction allows an increase of thetemperature difference between the heating elements and the solutionthus improving heat transfer. In addition, such temperature reduction isdesirable or may even be necessary when the product to be boiled issensitive to heat. As is known, when crystallizing sugar the conditionsfor circulating the solution forming the bath may be improved bychoosing a boiling temperature of about 70 C. and a constant degree ofsupersaturation lying between 1.1 and 1.3, the

viscosity of the sugar syrup then being at its minimum value.

EXAMPLE In order to obtain 1 t./h. of sugar crystals having an averagesize of 0.4 mm., a boiling apparatus as described above and shown inFIGS. 4 to 6 is used.

Seeding is effected in this case with sugar crystals having an averagesize of 0.03 mm. Assuming that the number of crystals remains constantin the apparatus, the number of crystals seeded per hour is 10 Forfeeding the bath, a sugar syrup is used which has saccharose purity anda supersaturation of 1.25. The total feed syrup flow rate is about 3t./h.

The evaporating chamber has a total length of 2.80 m. and six partitionwalls divide the bath into seven compartments of equal length (0.4 m.).The volume of the bath is 1 m The height and width of the bath ofsolution vary linearly between the entrance and the exit of theapparatus, the initial height and width being 0.275 and 0.185 in. andthe final height and width being 1.20 and 0.74 in. respectively.Saturated heating steam is used at 1.46 kg./ cm. C.) and the totalheating surface is about 13 m.

The evaporating chamber 1 is maintained at an absolute Compartment 21121 21 21.; 21 21 21 Q t./h V 0.10 0. 28 0. 36 0. 46 0. 51. 0. 58 0. 58S, (1111. 7 l3. 2 21. 5 32 4-1 58 74 11, dm. 28 54 87 128 176 232 205 I(lm. 9 35 (36 118 208 365 512 As is known, the purity of the feed syrupwhich is used plays an important role in crystallization. Thus, in theabove example, if syrup having a purity of 90% instead of 100% wereused, the duration of stay necessary for crystal growth is about threetimes as long. Consequently the flow rate of the crystals obtained withthe same apparatus would be /3 t./h. For a flow rate of 1 t./h., thelinear dimensions of the apparatus must be increased by the factor 3 /3while the heating surfaces may, however, remain the same.

Uniform crystal growth is obtained in the apparatus according to theinvention due to the fact that the crystals follow a helicoidal pathwith constant pitch in the bath of solution having a constant degree ofsupersaturation. One thus obtains a substantially linear variation ofthe average size of the growing crystals along the bath. Thus, for otherinitial and/ or final sizes than those given in the above example thelength of the apparatus will be decreased or increased to scale. Forfinal sizes greater than 0.4 mm., a corresponding section will be addedso as to lengthen the apparatus at the exit end.

On the other hand, for final sizes less than 0.4 mm. a correspondingsection will be removed so as to shorten the apparatus at this same end.In addition, if seeding is to be effected with crystals having a smallersize than 0.03 mm., the apparatus should be lengthened on the entranceside by means of a corresponding section.

We claim:

1. Apparatus for continuously crystallizing a substance from solution byevaporation, said apparatus comprising means defining an elongatedhorizontal evaporating chamber, said chamber inculding heating elements,solution inlets commmunicating with said chamber for continuouslysupplying a prescribed amount of feed solution along the 'bottom of saidfeed chamber, seed crystal feed means connected to one end of saidchamber, a discharge outlet provided at the other end of said chamberfor continuously discharging crystals having grown to a prescribed size,and means defining a vertical passageway communicating with saiddischarge outlet so as to provide an overflow at said other end of theevaporating chamber for maintaining a single bath at constant level ofsolution containing growing crystals advancing within said chambertowards said outlet, the bottom of said chamber being sloped downwardsand the width of said chamber increasing longitudinally, in thedirection of said other end, 0

such that the transverse cross-section of said constantlevel bathincreases substantially proportionally with the predetermined flow-rateof said bath advancing successively through each transverse bathcross-section, whereby to provide advancement of the bath at asubstantially constant horizontal rate along said chamber, said heatingelements being arranged along said chamber, within said bath so as toconstitute a series of vertical heating passages open at both oppositeends, extending transversely with respect to said chamber and on oneside thereof, for providing ascending movement of said bath byconvection within said passages on said one side of the chamber anddescending movement of said bath on the other opposed side of saidchamber, the heating area of said elements increasing along said seriesof passages towards said other end of said chamber, whereby saidconstant-level bath advances continuously in said chamber along a welldefined helicoidal path with substantially constant pitch for providinguniform growth of the crystals contained therein.

2. An apparatus according to claim 1 wherein the bottom of theevaporating chamber is inclined at an angle of 10 to 20 with respect tothe horizontal.

3. An apparatus according to claim 1 wherein said feed means for feedingthe evaporating chamber with feed solution comprises a longitudinal rowof inlet tubes discharging near the bottom of the chamber, the tubesbeing connected to a source of the solution, each via a valve forcontrolling the amounts of solution introduced by the tubes along thechamber.

4. An apparatus according to claim 1 wherein discharge means for thevapor released by evaporation comprise a condenser and a vacuum pumpconnected in series and a discharge orifice situated at the upper partof the evaporating chamber.

5. An apparatus according to claim 1 wherein said heating elements aresupplied with steam by means of at least one inlet tube provided with aregulating valve.

6. An apparatus according to claim 1 wherein said heating elements arearranged longitudinally in the chamber in two successive groups eachsupplied individually with steam.

7. An apparatus according to claim 1, further comprising a heatingjacket surrounding that part of the evaporrating chamber which includesthe bottom and the side-wall of the chamber, near which the ascendingmovement of the bath occurs.

8. An apparatus according to claim 1, further comprising a plurality oftransverse partition walls for subdividing said bath into a longitudinalseries of compartments communicating with each other, at the bottom ofthe evaporating chamber, by means of openings arranged at the lower endof said partition walls.

References Cited UNITED STATES PATENTS 3,424,221 1/1969 Luce l2716XFOREIGN PATENTS 555,139 6/1923 France 15925 1,324,801 3/1963 France127-16 JOSEPH SCOVRONEK, Primary Examiner D. G. CONLIN, AssistantExaminer U.S. Cl. X.R.

